1. FIELD OF THE INVENTION
This invention relates to holographic interferometry, and more particularly to such interferometry as used for the non-destructive testing of components.
2. BACKGROUND OF THE INVENTION
Holographic interferometric techniques have long been used for the non-destructive testing of industrial components. Typically, a first hologram is prepared of a component in an unstressed condition, a second hologram is prepared of the component in a stressed condition, and replicas of the wavefronts of light reflected from the object in its unstressed and stressed conditions reconstructed from the holograms are combined so that phase differences between them show up as patterns of interference fringes which characterize distortions of the component. In another technique, only a first hologram is prepared and its reconstructed wavefront is combined with light reflected directly from the object, so that the effect of the distortions can be viewed on a real time basis.
Various problems must be solved in implementing such techniques, depending on the applications. In every case, since the techniques depend upon the combination of wavefronts representing a test piece at different intervals in time, at least one of the wavefronts must be captured and stored in a hologram for a finite interval. Several techniques are known for such storage. Special photographic emulsions are capable of high resolution and sensitivity, but typically require a relatively slow development step which is usually difficult to perform in situ. This in turn creates problems in accurately locating the developed hologram at its original location. Two types of holographic recording apparatus address this problem. One uses photographic emulsions located in a liquid holding cell so that development with photographic chemicals can be done in situ. The other type uses photosensitive thermoplastic plates which are developed in situ by electrical and electronic means. In another technique, various phototropic materials are known which undergo short term changes in their optical properties on exposure to light and can thus be utilized for the short term storage of holograms, but use of such materials entails that the temporal separation between generation of successive holograms is matched to the response and storage properties of the medium, thus severely limiting the applicability of such systems. In a technique related to holography known as speckle interferometry, many systems rely upon electronic video cameras to capture images which can thereafter be manipulated electronically. Although video technology is constantly improving, such systems have traditionally been subject to the limited resolution of the images which can conveniently be stored and processed, limitations in the resolution of the original images having severe effects upon the information content of the resulting interferometric image. For this reason, optical methods of combining holographic wavefronts provide interferometric images of generally superior quality and information content.
In holographic interferometry systems, at least one of the wavefronts of the test piece must be reconstructed from a hologram. Ideally, this reconstruction makes use of the same laser that was originally utilized to produce the hologram. In practice, this may be difficult. Particularly in applications where it is desired to "freeze" motion of the test piece, e.g. where it is subject to vibration, the use of a high power pulsed laser to expose the hologram is desirable, but such lasers are not very suitable for subsequently reconstructing holographic images for observation since the short duration pulses make observation difficult. It is therefore common practice in such cases to utilize a different continuous wave laser for wavefront reconstruction and comparison, but since the laser used for reconstruction will commonly have a different wavelength, the optical system must be arranged to compensate for this, and there is inevitably some degradation of the interferometric data.